The current invention relates generally to phosphor blends having long persistence and tunable colors. More specifically, the current invention provides blends of one or more persistent phosphors with one or more other phosphors to create blends with long persistence and tunable colors.
A phosphor is a luminescent material that absorbs radiation energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. One important class of phosphors includes crystalline inorganic compounds of very high chemical purity and of controlled composition, to which small quantities of other elements, called “activators,” have been added for fluorescent emission. With the right combination of activators and inorganic compounds, the color of the emission of these crystalline phosphors can be controlled. Most useful phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic energy outside the visible range. Well known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (UV) radiation emitted by the excited mercury to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons, useful in photomultiplier tubes, or X-rays, such as scintillators used in imaging systems.
One important property of phosphors is the decay time, or the time required for the phosphor to stop emitting light after the excitation is removed. Most phosphor compositions have short decay times, with most of the stored energy emitted as light within seconds, or even a small fraction of a second, after excitation ends. Although their decay time is short, these phosphors have many possible color choices and multiple phosphor blends may be made with specific colors for use in lighting applications where continuous excitation is present.
New persistent phosphorescent materials have been developed in an attempt to extend the decay time for applications when continuous excitation is not present. However, many of these persistent phosphors have emission spectra that peak in the blue or green spectral regions, which may limit their visibility in certain applications. Accordingly, there is a need for new phosphorescent compositions that have long decay times and emission spectra that may be tuned for the application.